drm/i915/guc: Add LRC descriptor context lookup array

Add LRC descriptor context lookup array which can resolve the
intel_context from the LRC descriptor index. In addition to lookup, it
can determine if the LRC descriptor context is currently registered with
the GuC by checking if an entry for a descriptor index is present.
Future patches in the series will make use of this array.

v2:
 (Michal)
  - "linux/xarray.h" -> <linux/xarray.h>
  - s/lrc/LRC
 (John H)
  - Fix commit message

Cc: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721215101.139794-4-matthew.brost@intel.com